The electrophoretic effect is well known in the art as evidenced by the many patents and articles which describe the effect. As one of ordinary skill in the art will recognize, the electrophoretic effect operates on the principle that certain particles suspended in a medium can be electrically charged and thereby caused to migrate through the medium to an electrode of opposite charge. Electrostatic printing and electrophoretic image displays (EPID) use the electrophoretic effect to produce desired images. For an example of devices using the electrophoretic effect, reference is made to U.S. Pat. No. 4,732,830, issued to Frank J. DiSanto et al., on Mar. 22, 1988, entitled ELECTROPHORETIC DISPLAY PANELS AND ASSOCIATED METHODS, and assigned to Copytele, Inc., the assignee herein.
In typical EPIDs, dielectric particles are suspended in a fluid medium that is either clear or of an optically-contrasting color as compared with the dielectric particles. To effect the greatest optical contrast between the particles and the suspension medium, it is desirable to have either light-colored particles suspended in a dark medium or dark-colored particles, preferably black, suspended in a backlighted clear medium. A variety of pigment particle and dispersion medium compositions are known in the art. See, for example, U.S. Pat. No. 4,298,444, issued to K. Muller, et al., on Nov. 3, 1981, entitled ELECTROPHORETIC DISPLAY.
It has been found difficult to produce black and other very dark colored electrophoretic particles that are dielectric, of uniform size, and have a density matching that of a common suspension medium. As a result, EPIDs commonly use readily-manufactured light colored electrophoretic pigment particles suspended in media which contain dark color dyes. Such EPIDs are exemplified in U.S. Pat. Nos. 4,655,897 to DiSanto et al.; 4,093,534 to Carter et al.; 4,298,448 to Muller et al.; and 4,285,801 to Chiang. The use of a yellow pigment particle is disclosed in the following patents, all of which issued to Frederic E. Schubert and are assigned to Copytele, Inc., the assignee herein: U.S. Pat. No. 5,380,362, issued Jan. 10, 1995, entitled SUSPENSION FOR USE IN ELECTROPHORETIC IMAGE DISPLAY SYSTEMS; U.S. Pat. No. 5,403,518, issued Apr. 4, 1995, entitled FORMULATIONS FOR IMPROVED ELECTROPHORETIC DISPLAY SUSPENSIONS AND RELATED METHODS; and U.S. Pat. No. 5,411,656, issued May 2, 1995, entitled GAS ABSORPTION ADDITIVES FOR ELECTROPHORETIC SUSPENSIONS. These three patents are incorporated herein by reference.
The selection of the electrophoretic particles used in the EPID is very important in determining performance of the EPID and the quality of the image produced. Ideally, electrophoretic particles should have an optimum charge-to-mass ratio, which is dependent upon the particle size and surface charge; the optimum charge-to-mass ratio is desirable to obtain good electrostatic deposition at high velocity as well as rapid reversal of particle motion when voltages change.
There are advantages to polymerically coating electrophoretic particles for use in the EPID or other electrophoretic devices. A polymer coating applied to the particles can have the effect of enhancing the ability of the particles to scatter light. When polymer-coated particles impinge upon a transparent screen electrode (thereby displacing the fluid medium from the screen), a brighter color and sharper image is produced (as compared with when uncoated particles are used). Additionally, it naturally is desirable to use electrophoretic particles that are stable and resistant to interaction with the suspension medium to improve the efficiency and life-time of the EPID; the suspension medium may comprise, for example, aromatic or aliphatic solvents, including benzenes, toluenes, hydrocarbon oil, nonane, decane, or the like, which may interact over time with some typical uncoated pigment particles used in EPIDs. Polymer-coated pigment particles produce a harder and more solvent-resistant composite when compared to uncoated particles.
Furthermore, polymer-coated electrophoretic particles can be less apt to adhere to surfaces within the electrophoretic cell. Over recent years, attention has been directed to dispersion stabilization by adsorbed polymers on particle surfaces. See, for example, an article by P. Murau and B Singer, appearing in Vol 49, No. 9, of the Journal of Applied Physics (1978), entitled "The Understanding and Elimination of Some Suspension Instabilities in an Electrophoretic Display." See also U.S. Pat. No. 5,403,518, issued to Schubert, referenced above, and U.S. Pat. No. 4,285,801, issued to Anne A. Chiang on August 25, 1981, entitled ELECTROPHORETIC DISPLAY COMPOSITION.
It has been discovered that fluoro-polymers are advantageous for use as pigment particle coatings, as they have low critical surface tensions which tend to produce anti-stick properties. See, for example, an article by M. Hudlicry and A. E. Pavlath, appearing in Vol. 187, page 983, of ACS Monograph (1995), entitled "Properties of Fluorinated Compounds II." The lower surface tensions of perfluorinated polymers, when used as a pigment particle coatings, leads to less interaction between the pigment particle surfaces and the solvents comprising the suspension medium in which the particles are dispersed. The stabilizing polymer molecule hydrocarbon chains are more apt to extend out into the solvent and not sit on the particle surface because of its anti-stick character. This enhances the efficiency of the stabilizing mechanism.
There are, however, manufacturing problems associated with coating pigment particles. These problems involve the inability to coat all the pigment particles in a batch with a polymer and the formation of a percentage of polymer particles that have no pigment core.
Accordingly, a need exists for a method which enables all the pigment particles in a batch to be polymer coated while preventing the formation of polymer particles having no pigment core.